For most fibres used in the textile industry, such as synthetic fibres based upon polyesters, polyamides, polyacrylates and the like, or natural fibres such as wool and cotton, dyeing may be achieved by contacting fibres with a heated solution or dispersion of the dye, e.g. in a dye bath. Such methods are capable of achieving a strong colour hue and high dye substantivity (i.e. strong bonding of dye to fibres so that the dye has low susceptibility to leaching and transfer during washing). Another suitable method is to print colour onto textile formed from fibres, using a solution or dispersion (i.e. a suspension) of dye as a printing ink or paste.
Polypropylene, as a consequence of its high hydrophobicity, shows little or no affinity for dyes. As a consequence of this characteristic, the dyeing of polypropylene fibres by contacting the polypropylene fibres with dye solution or suspension/dispersion, or by printing, has, in the prior art, resulted in risk of poor dye substantivity, pale colouring and high risk of dye transfer problems.
To address such problems, polypropylene has, in the prior art, been coloured by addition of pigments, such as organic and/or inorganic pigments, to the polypropylene as a bulk melt, prior to fibre formation, so that the pigments are physically entrapped within the polypropylene fibre bodies following re-solidification after fibre formation from the polypropylene melt. Typically, polypropylene fibres may be formed by melt extrusion of the polypropylene. As is common for polymer process methods, the pigments may be provided in the form of a masterbatch blended with polypropylene chips or pellets prior to formation of the melt. The term “masterbatch”, as used in this field refers to a composition, usually in the form of solid chips or pellets, having a polymer as a base into which additive(s) may be dispersed and/or dissolved. Pellets of masterbatch are then mixed with polymer chips or pellets and melted together so that the desired additives may be uniformly and homogeneously dissolved and/or dispersed throughout the resulting melt. Usually, the polymer used as base for the masterbatch may be the same polymer or polymer type as the polymer into which the additive(s) are to be blended.
Such a bulk process is only economically viable for large production batches only. This may limit the flexibility in the choice of colours for the resulting fibres and articles made from the fibres.
Hence, there is a need for methods and compositions to modify polypropylene in a manner so that polypropylene fibres may be more capable of being dyed by contacting with dispersions and/or solutions of dyes. In particular, it would be desirable to be able to employ so-called disperse dyes for the dyeing of polypropylene fibres, as such dyes are readily available in deep colours.
The addition of polar polymers, containing acid or basic groups, blended with polypropylene for use as dye substantivity enhancer was described in U.S. Pat. Nos. 3,328,484, 3,433,853, 3,465,060 and 3,686,848. These disclosures relate to polypropylene modified to be dyeable by means of acid or basic dyes.
EP-A-0468519 discloses blending of co-polyamide into melt polypropylene prior to fibre formation in order to render the polypropylene fibres dyeable by means of subsequent contacting with acidic dyes.
International patent publication WO 2006/056690 discloses the use of a thermoplastic composition as an additive to polypropylene, in combination with an antioxidant, for improving dyeability or printability. The thermoplastic composition is described as a graft copolymer containing polyamide blocks, having a functionalised polyolefin backbone selected from copolymers of ethylene maleic anhydride and ethylene (meth)acrylate maleic anhydride. It is disclosed that the resulting, so-called alloys of polyolefin (i.e. polypropylene) and graft copolymer, are for use with polar molecules as inks or dyes.